TERAHERTZ STUDIES OF COLLISION-BROADENED ROTATIONAL LINES

Via terahertz (THz) coherent transients and THz time-domain spectrosco py, we have measured the far-wing absorption line profile of the ensem bles of collision-broadened ground state rotational lines of methyl br omide, methyl chloride, and methyl fluoride vapors out to more than 20 0 line widths from resonance, corresponding to frequency offsets as mu ch as 5 times the resonant frequency. On these far wings the measured absorption is approximately an order of magnitude less than that predi cted by the van Vleck-Weisskopf theory. Our observations show that as the offset frequency is increased, a transition occurs from the regime of the van Vleck-Weisskopf theory to the regime of the Lorentz theory . These measurements are fit to a new molecular response theory which explicitly includes the molecular orientation time during a collision. Due to the broad bandwidth of the THz pulses, we demonstrate the vali dity of this molecular response theory for the far-wing absorption of methyl fluoride, chloride, and bromide. The excellent theoretical fit to our measurements encompassing the frequency range over which this t ransition occurs indicates a molecular response time on the order of 2 00 fs. These measurements also permit determination of the line-width dependence on the rotational quantum number J.